TWI510650B - Lead - free steel - Google Patents

Description

Lead-free steel

This invention relates to quick-cutting steels and, more particularly, to lead-containing, fast-cut steels containing lead.

General mechanical articles such as automobiles and electrochemical products contain a plurality of parts. Most of these parts are manufactured by cutting. Therefore, steel as a part material is required to be "easy to cut", in other words, it has excellent machinability.

The machinability of the quick-cut steel is excellent. Typical quick-cut steels include, for example, SUM23 and SUM24L specified in Japanese Industrial Standard JIS Standard. Pb can improve the machinability of steel, so most of the fast-cut steel contains Pb. Hereinafter, the fast-cut steel containing Pb is referred to as lead-containing quick-cut steel.

In recent years, based on considerations of the environment, a technical solution for reducing the Pb content of the fast-cut steel and the Pb-free lead-free quick-cut steel has been proposed. However, the machinability is superior to lead-free steel. Therefore, even now, the demand for lead-free quick-cut steel is still high. Recently, the surface quality of parts, such as the shape of the part and the thickness of the surface, is required to be higher. Precision. Therefore, even for lead-containing quick-cut steel, it is required to improve the machinability.

It has been known in the past that it is machinability as long as it contains Pb. It can be improved. However, the report case for the existence form of Pb in steel is hardly seen. Moreover, the above-mentioned low carbon lead-containing quick-cut steel SUM24L contains Pb, S and P. However, even if the SUM24L is not sufficiently machined, there is a case where the desired surface thickness cannot be obtained. In addition, if the chemical composition corresponding to SUM24L contains S and P for improving the machinability, the machinability can be improved, but it is easily broken in the manufacturing process.

Japanese Patent Publication No. 11-222646 (Patent Document 1) The technical solution disclosed in Japanese Laid-Open Patent Publication No. 2004-176175 (Patent Document 2) is to improve the machinability of the fast-cut steel. Specifically, as disclosed in Patent Document 1 and Patent Document 2, the machinability of steel is improved by controlling the form of MnS inclusions in the steel.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. Hei 11-222646

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2004-176175

However, in the case of lead-containing quick-cut steel, if the form of the MnS inclusions is simply controlled, sometimes sufficient machinability cannot be obtained.

An object of the present invention is to provide a lead-containing quick-cut steel excellent in machinability.

The lead-containing quick-cut steel of the present embodiment contains, in mass%, C: 0.005 to 0.2%, Mn: 0.3 to 2.0%, P: 0.005 to 0.2%, S: 0.01 to 0.7%, and Pb: 0.03 to 0.5. %, N: 0.004~0.02%, and O: 0.003~0.03%, and the rest is composed of Fe and impurities. Further, the number of Pb inclusions corresponding to a circle diameter of 0.01 to 0.5 μm in the steel is 10,000 pieces/mm ² or more.

The lead-containing quick-cut steel of the present embodiment has excellent machinability.

More preferably, in the lead-containing fast-cut steel, the total amount of Pb inclusions in the steel corresponding to a circle diameter of 0.01 to 0.5 μm and the number of MnS inclusions corresponding to a circle diameter of 0.01 to 0.5 μm are 15,000 pieces / mm ² .

The lead-containing quick-cut steel may contain one or more selected from the group consisting of Cu: 0.5% or less, Ni: 0.5% or less, and Sn: 0.5% or less, in place of Fe. portion. In addition, the lead-containing quick-cut steel may contain one or more selected from the group consisting of Te: 0.2% or less and Bi: 0.5% or less, in place of a part of Fe. Further, the above-mentioned quick-cut steel may contain one or more selected from the group consisting of Cr: 0.5% or less and Mo: 0.5% or less, in place of a part of Fe.

1‧‧‧Steel

2‧‧‧Car blade edge

3‧‧‧Cutting tools

4‧‧‧Pb inclusions

5‧‧‧MnS inclusions

6‧‧‧Pb

7‧‧‧Pb-MnS inclusions

20‧‧‧cutting tools

30‧‧‧ round bars

40‧‧‧Micro Pb inclusions

100‧‧‧Substrate

G1~G10‧‧‧Ditch

Fig. 1A is a cross-sectional view showing the vicinity of a cutting surface when the edge portion of the turning edge of the turning edge is large at the time of cutting.

Fig. 1B is a cross-sectional view showing the vicinity of the cutting surface in the case where the edge portion of the turning edge of the turning edge is small at the time of cutting.

Fig. 2 is a photographic image of Pb inclusions and Pb-MnS inclusions in steel.

Figure 3 is a photographic image of fine Pb inclusions in the substrate.

Fig. 4 is a schematic view for explaining the cooling rate in the casting process.

Fig. 5A is a schematic view for explaining a flange cutting test.

Fig. 5B is another schematic view for explaining the flange cutting test.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The same or equivalent parts in the drawings are denoted by the same element symbols, and the description thereof is not repeated. The "%" of the content of the element described below means: mass%.

The present inventors focused on the relationship between the form of Pb inclusions and the MnS inclusions in the lead-containing quick-cut steel and the machinability, and investigated and reviewed them. As a result, the inventors obtained the following novelty.

(A) As long as the machinability of steel is high, it is cut and added. The surface roughness of the steel after work can be good, and the life of the cutting tool can be extended. The machinability is affected by the "built-up edge" of the cutting edge of the cutting tool attached to the cutting edge of the cutting tool.

The so-called "built-up edge" It means that it is part of the steel material being cut, and it is also the chip of the cutting edge of the cutting tool attached to the cutting process. In the cutting, the edge portion of the turning edge of the turning edge is a function of gradually cutting off the cutting tool and adhering to the cutting tool, and functions as a substantial cutting edge. Therefore, the edge of the cutting edge of the turning edge affects the machinability.

Figure 1A and Figure 1B show the cutting process A cross-sectional view of the vicinity of the cutting face after the cutting tool is removed. The white dashed line in the figure represents the position of the cutting edge of the cutting tool 3. In Fig. 1A, a large blade edge portion 2 is formed, and the blade edge portion 2 is detached from the cutting tool 3 and adheres to the steel material 1. On the other hand, the cutting edge portion of the turning edge in Fig. 1B is much smaller than the cutting edge portion 2 of the turning edge of Fig. 1A, so that the cutting tool 3 and the steel material 1 are separated.

As mentioned above, as long as the edge of the cutting edge of the turning tool grows sharply If the edge of the cutting edge of the turning edge is easily attached to the steel. The edge of the cutting edge attached to the steel will be in contact with the cutting tool again. In this case, sometimes the cutting tool will be damaged. In addition, the surface roughness of the cutting surface of the steel material sometimes becomes thick due to the influence of the edge portion of the cutting edge of the steel blade attached to the steel material. In addition, when the edge of the cutting edge of the turning edge is detached from the cutting tool, sometimes there is still a part of the cutting edge product. The edge of the chip will remain in the cutting tool. In this case, a part of the edge portion of the remaining blade edge will become the core, and the edge of the blade edge will be grown again. Therefore, the cutting tool will be damaged and the surface of the steel will become thicker.

On the other hand, the cutting edge of the turning tool as shown in Fig. 1B When the edge is small, the edge of the blade edge is easily detached from the steel and the cutting tool. In this case, the edge of the cutting edge of the turning edge does not easily affect the life of the cutting tool, and the surface thickness of the steel tends to be good (the surface thickness is small).

As mentioned above, the smaller the edge of the cutting edge of the turning tool, the better. When cutting, the less the edge of the blade edge is, the easier it is to grow. When the edge of the turning edge of the turning edge is small, the crack generated by the falling off of the edge of the cutting edge of the turning tool can be promoted. In addition, the edge of the blade edge of the turning edge is frequently peeled off in a fine state, so that the surface thickness tends to be good and the tool life is also prolonged. That is, the machinability can be improved.

(B) Figure 2 is a lead obtained by high-power microscope observation. A photo of a section of a fast-cut steel. It can be seen from Fig. 2 that there are substrates in the lead-free steel: substrate 100, Pb inclusions 4, MnS inclusions, and Pb-MnS inclusions 7. In the present specification, the term "Pb inclusion 4" means an inclusion composed of Pb and impurities. The term "MnS inclusion" means an inclusion composed of Mn, S, and impurities. The term "Pb-MnS inclusions 7" means inclusions containing MnS inclusions 5 and Pb6 adhering to the surface of the MnS inclusions 5. In this specification, these three types of inclusions are collectively referred to as "fast-cut inclusions".

Profile in the direction of extension of the steel (eg rolling direction) In the above, each of the inclusions (Pb inclusion 4, MnS inclusion, and Pb-MnS inclusion 7) corresponds to a circle diameter and is larger than 0.5 μm. Hereinafter, Pb inclusions having a circle diameter of more than 0.5 μm, MnS inclusions, and Pb-MnS inclusions are referred to as "coarse fast-cut inclusions". Large and fast-cut inclusions will cause stress concentration during cutting to promote cracking and progress. The smaller the aspect ratio of the coarse and fast-cut inclusions and the more spheroidal, the more likely the stress concentration will be, and the more likely the cracks will progress and progress.

(C) On the other hand, in the substrate 100, in the extension of the steel In the cross section of the extending direction, there is a Pb inclusion corresponding to a circular diameter of 0.5 μm or less. Hereinafter, Pb inclusions having a circle diameter of 0.01 to 0.5 μm in a cross section in the direction in which the steel material is extended are referred to as "fine Pb inclusions".

Figure 3 is the implementation of the method using the copy extraction method. A photographic image of the fine Pb inclusions 40 in the substrate 100 of the lead-containing fast-cut steel. As can be seen from Fig. 3, in the substrate 100, spherical fine Pb inclusions 40 having a small aspect ratio are dispersedly present.

Micro Pb inclusions can cause embrittlement of the substrate. Therefore, if a large amount of fine Pb inclusions are dispersed in the substrate, the edge of the blade edge will not grow coarsely, and it is easy to repeatedly generate and fall off the edge of the edge of the blade. . As a result, the machinability of lead-containing quick-cut steel can be improved. Specifically, if the number of fine Pb inclusions is 10,000 pieces/mm ² or more, excellent machinability can be obtained.

(D) If a large amount of MnS inclusions having a diameter of 0.01 to 0.5 μm in the cross section of the steel material are present together with the fine Pb inclusions in the substrate, a more excellent result can be obtained. Machinability. In the following, the MnS inclusions having a circle diameter of 0.01 to 0.5 μm in the cross section in the direction in which the steel material is extended are referred to as "fine MnS inclusions". Although the embrittlement effect of the fine MnS inclusions is not as good as that of the fine Pb inclusions, the substrate can be embrittled. Therefore, not only the fine Pb inclusions, but also if a large amount of fine MnS inclusions are dispersed in the substrate, the machinability can be further improved. Specifically, if the total number of fine Pb inclusions and the number of fine MnS inclusions is 15,000 pieces/mm ² or more, the machinability of the lead-containing quick-cut steel can be further improved.

Based on the above findings, the inventors, etc., completed this The lead-containing quick-cut steel of the embodiment. The lead-containing quick-cut steel of the present embodiment will be described in detail below.

[chemical composition]

The lead-containing quick-cut steel of the present embodiment has the following chemical composition points.

C: 0.005~0.2%

Carbon (C) can increase the strength of steel. C is more affected by the oxygen content in the steel and the machinability. If the C content is too low, a large amount of oxygen remains in the steel and pores may occur. In addition, a hard oxide is formed and falls. Low machinability. On the other hand, if the C content is too high, the strength of the steel becomes too high and the machinability is lowered. Therefore, the C content is 0.005 to 0.2%. A good lower limit of the C content is more than 0.005%, more preferably 0.05%, still more preferably 0.07%. The upper limit of the C content is preferably less than 0.2%, more preferably 0.12%, still more preferably 0.09%.

Mn: 0.3~2.0%

Manganese (Mn) forms a soft oxide in molten steel and suppresses the formation of hard oxides. Therefore, the machinability of steel will increase. Mn can be combined with S to form MnS, which can reduce the amount of solid solution S. When the amount of solid solution S is reduced, high temperature embrittlement cracking can be suppressed. When the Mn content is too low, it is difficult to obtain the above effects. If the Mn content is too low, S will form FeS instead of forming MnS, and the steel will be brittle. On the other hand, if the Mn content is too high, the hardness of the steel will become too high, and the machinability and cold workability will be lowered. Therefore, the Mn content is 0.3 to 2.0%. A good lower limit of the Mn content is more than 0.3%, more preferably 0.5%, still more preferably 0.8%. A good upper limit of the Mn content is less than 2.0%, more preferably 1.8%, and even more preferably 1.6%.

P: 0.005~0.2%

燐 (P) can embrittle steel and improve the machinability of steel. If the P content is too low, this effect cannot be obtained. On the other hand, if the P content is too high, the effect of improving the machinability will be saturated. If the P content is too high, it will be more difficult to stably produce steel. Therefore, the P content is 0.005~ 0.2%. A good lower limit of the P content is more than 0.005%, more preferably 0.03%, still more preferably 0.05%. A good upper limit of the P content is less than 0.2%, more preferably 0.15%, still more preferably 0.1%.

S: 0.01~0.7%

Sulfur (S) is combined with Mn to form MnS inclusions. MnS inclusions improve the machinability of steel. Further, Pb is a periphery of MnS which is agglomerated during the solidification process, and therefore MnS uniformly disperses Pb in steel. If the S content is too low, the above effects cannot be obtained. On the other hand, if the S content is too high, a sulfide having a coarse MnS as a main component is formed, and the deformation property between heat is lowered. Therefore, the S content is 0.01 to 0.7%. The lower limit of the S content is preferably more than 0.01%, more preferably 0.05%, still more preferably 0.15%, based on the balance between the machinability and the manufacturability of rolling or the like. A good upper limit of the S content is less than 0.7%, more preferably 0.5%, and even more preferably 0.4%. If it is based on the maintenance of the quality of the steel at the time of manufacture and the mechanical properties other than the machinability, the good S content is 0.28% or more.

Pb: 0.03~0.5%

Lead (Pb) is Fe which hardly dissolves in the substrate, but forms soft Pb inclusions. Pb will then adjoin the periphery of the MnS to form Pb-MnS inclusions. Pb will be present in the form of fine Pb inclusions in the substrate, which can improve the machinability of the steel. If the Pb content is too low, it cannot be obtained. Said effect. On the other hand, if the Pb content is too high, it is difficult to stably produce a lead-containing quick-cut steel. Therefore, the Pb content is 0.03 to 0.5%. A good lower limit of the Pb content is more than 0.03%, more preferably 0.1%, still more preferably 0.15%. A good upper limit of the Pb content is less than 0.5%, more preferably 0.4%, and even more preferably 0.35%.

N: 0.004~0.02%

Nitrogen (N) affects the machinability and the surface thickness after cutting. When the N content is too low, the index in the steel during cutting tends to vary. Therefore, the ductility of the substrate becomes too high. In this case, it becomes easy to generate chip ridges, and a good surface roughness cannot be obtained. On the other hand, if the N content is too high, the index becomes less likely to change. In this case, the steel is brittle, and the steel becomes easily cracked during cold working other than cutting such as drawing (stretching steel wire) and cold forging. Therefore, the N content is 0.004 to 0.02%. A good lower limit of the N content is more than 0.004%, more preferably 0.006%, and even more preferably 0.008%. A good upper limit of the N content is less than 0.02%, more preferably 0.018%, still more preferably 0.015%.

O: 0.003~0.03%

Oxygen (O) can affect the shape of MnS. When the O content is very low, the oxygen content in the MnS is also reduced. Therefore, the extensibility of MnS will increase. When steel is processed by rolling or the like, MnS can easily extend in a specific direction (for example, rolling direction), and it is easy to be in the steel body. There is an anisotropy on it. In this case, at the time of cutting, the edge portion of the cutting edge of the turning edge tends to be enlarged, and the steel portion after the cutting is irregularly peeled off. Therefore, the surface of the steel becomes thick and the tool deteriorates. In the present embodiment, in particular, the shape of MnS affects the dispersion of Pb. Therefore, the MnS having a high aspect ratio (in other words, already extended) is not good. On the other hand, when the O content is too high, an excessive amount of hard oxide is formed in the steel, and the machinability of the steel is lowered. Therefore, the O content is 0.003 to 0.03%. A good lower limit of the O content is more than 0.003%, more preferably 0.005%, still more preferably 0.008%, still more preferably 0.012%. A good upper limit of the O content is less than 0.03%, more preferably 0.025%, and even more preferably 0.022%. In the case of considering the melting loss of the refractory or the like, the upper limit of the more preferable O content is 0.018%.

The remainder of the lead-containing quick-cut steel of the present embodiment is composed of iron (Fe) and impurities. The term "impurity" as used herein refers to an ore that is used as a raw material for steel, a recycled waste, or an element that is mixed from the environment of the manufacturing process.

[About fine Pb inclusions]

In the quick-cutting steel of the present embodiment, the number N _Pb of Pb inclusions (fine Pb inclusions) corresponding to a circular diameter of 0.01 to 0.5 μm in the cross section of the steel material in the extending direction is 10,000 pieces/mm ² or more. As described above, the substrate is embrittled by a large amount of dispersion of fine Pb inclusions in the substrate. Therefore, at the time of cutting, the edge portion of the fine turning edge of the turning edge is frequently generated and fallen off. As a result, the machinability will increase. If the number of fine Pb inclusions N _{Pb is} less than 10,000 / mm ² , the substrate is not sufficiently embrittled. Therefore, the formation and disengagement of the edge portion of the cutting edge of the turning edge becomes a shape resulting from the coarse and fast cutting inclusions. If there is a large and fast-cut inclusion in the steel with a large aspect ratio (in other words, it has been extended), the material of the steel portion containing the coarse inclusions tends to be uneven. Therefore, the attachment, generation, and growth of the edge of the blade edge of the blade can easily cause unevenness in the width direction of the cutting edge. In this case, the edge of the blade edge of the turning edge has a large unevenness, and it is easy to become coarse. As a result, the shape of the peeling piece at the edge of the blade edge of the turning edge is irregular and coarse, which causes damage to the tool or causes deterioration of the surface thickness. That is, the machinability is lowered.

The number N _Pb of the fine Pb inclusions is preferably 15,000 pieces/mm ² or more, more preferably 20,000 pieces/mm ² or more. The upper limit of the number of fine Pb inclusions N _Pb is not particularly limited. The upper limit of the number N _Pb of the fine Pb inclusions is, for example, 1,000,000 pieces/mm ² .

[About fine MnS inclusions]

More preferably, the number of the fine Pb inclusions and the total number of MnS inclusions (fine MnS inclusions) having a diameter of 0.01 to 0.5 μm in the cross section of the steel material extending direction (hereinafter referred to as : The total number of finely-cut inclusions TN) is 15,000 pieces/mm ² or more. Although the effect of the fine MnS inclusions is smaller than that of the fine Pb inclusions, the substrate can be embrittled. Therefore, when the total number TN of the finely-curved inclusions is 15,000 pieces/mm ² or more, the substrate can be made more brittle and the machinability can be further improved.

The total number TN of the finely-cured inclusions is preferably 20,000 / mm ² or more, more preferably 25,000 / mm ² or more. The upper limit of the total number TN of the finely-cured inclusions is not particularly limited. The upper limit of the total number TN of the finely-curved inclusions is, for example, 1,000,000 pieces/mm ² .

[Method of measuring the number of fine Pb inclusions N _Pb and the total number of finely-cut inclusions TN]

The number N _Pb of fine _Pb inclusions and the total number TN of finely-cut inclusions were determined according to the following measurement methods. First, a cross section parallel to the direction in which the lead-containing fast-cut steel (for example, bar steel, wire, etc.) is stretched, and including the center line of the lead-containing fast-cut steel (hereinafter, referred to as a main surface) ). On the main surface, a test piece is taken from the surface of the lead-containing fast-cut steel, and the depth position (so-called R/2 position) corresponding to the 1/2 radius in the diameter direction. Further, a sample was prepared from the main surface of the test piece according to the extraction and replication method. A TEM image of any 10 fields of view in the surface of the sample was taken using a transmission electron microscope (TEM). The magnification of the TEM is set to 20,000 times. The area of each field of view was set to 50 μm ² (10 μm × 5 μm; that is, 5 × 10 ^-4 mm ² ).

Inclusions are identified in each field of view. Specifically, the identification of inclusions is performed using an EPMA (Electron Microanalyzer) or an EDS (Energy Dispersive X-ray Microanalyzer). In this way, Pb inclusions and MnS inclusions can be defined.

In addition, the equivalent circle of each inclusion in each field of view is also obtained. diameter. The equivalent diameter of the circle refers to the diameter of the circle when the area of the inclusion is converted into a circle of the same area. Corresponding to the circular diameter, it is possible to perform measurement using a TEM image and using a well-known particle size distribution measurement software.

According to the above measurement, the total number N1 (pieces) of Pb inclusions (fine Pb inclusions) corresponding to a circle diameter of 0.01 to 0.5 μm in 10 fields of view and the corresponding circle diameter in 10 fields of view were determined to be 0.01. The total number of MnS inclusions (fine MnS inclusions) of ~0.5 μm is N2 (number). Then, the number of fine Pb inclusions N _Pb (pieces/mm ² ) and the total number of finely-cut inclusions TN (pieces/mm ² ) were determined according to the following formula (1) and the formula ( ² ).

N _Pb =N1/TA (1)

TN=(N1+N2)/TA (2)

Here TA (mm ² ) is the total area of 10 fields of view. Under the above conditions, TA = 5 × 10 ^-4 (mm ² ).

[about selection elements]

The lead-containing quick-cut steel of the present embodiment may further contain one or more selected from the group consisting of Cu, Ni, and Sn instead of a part of Fe. These optional elements increase corrosion resistance.

Cu: 0.5% or less

Copper (Cu) is a selective element. Cu can improve the corrosion resistance of steel. Cu can also improve the machinability of steel. On the other hand, the Cu content is too high In this case, the thermal ductility of the steel will decrease. Therefore, the Cu content is 0.5% or less. When the Cu content is 0.05% or more, the above effects can be remarkably obtained. A more preferable lower limit of the Cu content is 0.07%, and more preferably 0.15%. A good upper limit of the Cu content is less than 0.5%, more preferably 0.4%, still more preferably 0.3%.

Ni: 0.5% or less

Nickel (Ni) is a selective element. Ni improves the corrosion resistance of steel. Ni can also increase the ductility of steel. If the lead-containing fast-cut steel contains Cu, Ni can suppress the embrittlement of the lead-containing fast-cut steel and improve the manufacturing stability of the steel. On the other hand, if the Ni content is too high, the ductility becomes too high and the machinability is lowered. Therefore, the Ni content is 0.5% or less. When the Ni content is 0.05% or more, the above effects can be remarkably obtained. A more favorable lower limit of the Ni content is 0.1%. A good upper limit of the Ni content is less than 0.5%, more preferably 0.4%, still more preferably 0.3%.

Sn: 0.5% or less

Tin (Sn) is a selective element. Sn can improve the corrosion resistance of steel. Sn can also improve the machinability of steel. On the other hand, if the Sn content is too high, the thermal ductility of the steel is lowered. Therefore, the Sn content is 0.5% or less. When the Sn content is 0.05% or more, the above effects can be remarkably obtained. A more preferable lower limit of the Sn content is 0.1%, more preferably 0.2%. A good upper limit of the Sn content is less than 0.5%, more preferably 0.4%, still more preferably 0.3%.

The lead-containing quick-cut steel of the present embodiment may also contain Te from And one or more selected from the group consisting of Bi, in place of a part of Fe. These elements are optional elements that improve the machinability of the steel.

Te: 0.2% or less

碲 (Te) is a selection element. Te improves the machinability of steel. Te is particularly effective for shape control of the fast-cut inclusions, and specifically, the aspect ratio of the MnS inclusions and the Pb-MnS inclusions can be made small. On the other hand, if the Te content is too high, the thermal ductility of the steel is lowered. Therefore, the Te content is 0.2% or less. When the Te content is 0.0003% or more, the above effects can be remarkably obtained. A more preferable lower limit of the Te content is 0.0008%, and more preferably 0.01%. A good upper limit of the Te content is less than 0.2%, more preferably 0.1%, still more preferably 0.05%.

Bi: 0.5% or less

铋 (Bi) is a selection element. Bi improves the machinability of steel. On the other hand, if the Bi content is too high, the thermal ductility of the steel may be lowered. Therefore, the Bi content is 0.5% or less. When the Bi content is 0.005% or more, the above effects can be remarkably obtained. A more preferable lower limit of the Bi content is 0.008%, more preferably 0.01%. A good upper limit of the Bi content is less than 0.5%, more preferably 0.1%, still more preferably 0.05%.

The lead-containing quick-cut steel of the present embodiment may contain one or more selected from the group consisting of Cr and Mo instead of a part of Fe. These optional elements increase the hardness of the rolled steel.

Cr and Mo can improve quench hardenability. So even The low carbon steel of the lead-containing quick-cut steel of the present embodiment can sometimes be effectively used to adjust the strength of the material after rolling. In the lead-containing quick-cut steel of the present embodiment, the material subjected to work hardening by a drawing wire (drawing a steel wire) is often cut. In general, the harder the steel is, the better the surface roughness, but it will promote the wear of the tool. Therefore, the hardness of the steel will affect the dimensional accuracy. In the case of precision parts, it is preferable to control the hardness of the steel which has been subjected to work hardening by the drawing of the wire to 150 to 250 HV, and it is most appropriate to adjust the shape and the amount of cutting to be processed. The hardness is suitable.

The hardness of the steel after work hardening after stretching the wire is It depends on the hardness, work hardening characteristics and processing amount of the steel after rolling. When the amount of processing (for example, the rate of reduction of the wire) is small, the hardness after processing is not easily increased. Therefore, it is effective to increase the hardness of the rolled steel in advance. Therefore, it is effective to add such an element capable of improving quench hardenability to Cr and/or Mo.

Cr: 0.5% or less

Chromium (Cr) is a selective element. Cr can increase the hardness of the steel after rolling. If the Cr content is too high, the steel becomes too hard, and it is difficult to obtain the machinability as a fast-cut steel. Therefore, the Cr content is 0.5% or less. When the Cr content is 0.05% or more, the above effects can be remarkably obtained. A good lower limit of the Cr content is 0.08%, more preferably 0.1%. A good upper limit of the Cr content is less than 0.5%, more preferably 0.3%, and even more preferably 0.2. %.

Mo: 0.5% or less

Molybdenum (Mo) is a selective element. Mo can increase the hardness of the steel after rolling. If the Mo content is too high, the steel becomes too hard, and it is difficult to obtain the machinability as a fast-cut steel. Therefore, the Mo content is 0.5% or less. When the Mo content is 0.02% or more, the above effects can be remarkably obtained. A good lower limit of the Mo content is 0.03%. A good upper limit of the Mo content is less than 0.2%, more preferably 0.1%.

〔Production method〕

Next, an example of the above-described method for producing lead-containing quick-cut steel will be described.

First, a molten steel that meets the chemical composition described above is produced into a cast piece by a continuous casting method. Alternatively, the molten steel is produced into a cast bead by a bulking method (casting step). Then, the cast piece or the cast piece is subjected to hot work to produce a lead-containing quick-cutting steel material (heat-process processing step). Each process will be described in detail below.

[casting process]

In the casting process, molten steel is cast to produce a cast piece. The cast piece is one of three types of cross-sectional areas of, for example, 350 mm × 560 mm, 220 mm × 220 mm, and 150 mm × 150 mm. The cooling rate RC of the molten steel is controlled according to the sectional area of the material and the cooling conditions during solidification. The solubility of Pb in molten steel is almost zero, and it is liquid in molten steel. The state of the drops is scattered. At the time of solidification, Pb aggregates with the MnS inclusions to form coarsely-cut inclusions (Pb-MnS inclusions), or the Pb particles aggregate with each other to form coarse Pb inclusions. In addition, Pb also generates fine Pb inclusions. The molten steel is sufficiently stirred, and by controlling the cooling rate RC at the time of solidification, the fine Pb inclusions are largely dispersed in the steel.

Figure 4 is a cross-sectional view of the cast piece after casting. In thickness In the slab of W (mm), the cooling rate from the liquidus temperature to the solidus temperature is defined as the casting speed at the position P1 of W/4 from the surface toward the center of the material. The cooling rate RC (° C/min) in the step S1. If the cooling rate RC is 15 to 30 ° C / min, the fine Pb inclusions are dispersed in a large amount in the steel.

If the cooling rate RC is less than 15 ° C / min, Pb will settle due to solidification too slowly, or agglomerate around the MnS inclusions to form coarse Pb-MnS inclusions. Therefore, the number N _{Pb of} fine Pb inclusions will be less than 10,000 / mm ² .

On the other hand, if the cooling rate RC exceeds 30 ° C / min If so, the solid solution S will increase excessively. As a result, the thermal ductility of the steel is lowered. Therefore, in the case of manufacturing a material (cast piece) by the continuous casting method, there is sometimes a problem of breakout. In addition, in the hot room processing, sometimes the material may crack or cause flaws caused by cracks.

The cooling rate RC can also be obtained by the following method. The solidified material is cut in the transverse direction. Among the cross sections of the material, The interval λ2 (μm) of the secondary dendrite arms in the thickness direction of the solidified structure at the point P1 was measured. Using the measured value λ2, the cooling rate RC (° C/min) was obtained from the following equation (3).

RC=(λ2/770) ^-(1/0.41) (3)

The interval λ2 of the secondary dendritic arms is dependent on the cooling rate. Therefore, the cooling rate RC can be obtained by measuring the interval λ2 of the dendrite arm twice.

Further, at the time of continuous casting, the molten steel is sufficiently stirred. Specifically, at the time of continuous casting, the molten steel in the mold is stirred, and the molten steel flow rate VE is set to 10 to 40 cm/s.

If the molten steel flow rate VE is less than 10 cm/s, the stirring is insufficient. Therefore, fine Pb inclusions are less likely to be formed and are not easily dispersed uniformly, and the number of Pb inclusions N _Pb does not reach 10,000 pieces/mm ² . On the other hand, if the molten steel flow rate VE exceeds 40 cm/s, the variation of the molten steel noodle surface becomes too large, and continuous casting becomes difficult.

As described above, by controlling the molten steel flow rate VE and the cooling rate RC, the number N _{Pb of the} fine Pb inclusions can be controlled to 10,000 pieces/mm ² or more.

In the above casting step, it has been described that the production is carried out by continuous casting. However, the augmentation method can also be used to manufacture the cast embryo. In this case, a mold having a sectional area of 40,000 mm ² or less (for example, 200 mm × 200 mm) is used, and the agglomeration is carried out by an upper injection method. In this case, the molten steel is stirred at a speed corresponding to a flow rate VE of molten steel of 10 to 40 cm/s, and the cooling rate RC also tends to be 15 to 30 ° C / min.

[heat processing process]

In the hot working process, the material is first heated. Then, the heated material is subjected to hot working to produce a lead-containing fast-cut steel. Lead-containing fast-cutting steels are, for example, steel bars, wires, and ingots. The inter-heat processing includes, for example, block rolling, continuous rolling using a V-H machine, hot forging, and the like.

In the hot working process, the surface temperature of the material at the start of the hot working (hereinafter referred to as the processing start temperature) is set to 1000 ° C or higher. When the processing start temperature is low, the fine Pb inclusions are excessively concentrated and do not uniformly disperse, and the number of Pb inclusions N _Pb cannot be formed to 10,000 pieces/mm ² or more.

In addition, fine MnS inclusions are generated in large amounts during hot processing. When the processing start temperature is less than 1000 ° C, the fine MnS inclusions may not be sufficiently formed. In this case, sometimes the total number of finely-cutted inclusions TN cannot reach 15,000/mm ² .

In the hot intercalation process, there are cases where a plurality of hot intercalations are performed. For example, after the material is heated, the block rolling (the first hot-storing process) is performed, and then the material after the block rolling is heated again, and the product is rolled to produce a steel bar (the second time). Cases such as hot processing. In this case, when the processing start temperature at the time of the hot-spinning process (at the time of the first hot-spinning process) is set to 1000 ° C or more, the number of fine Pb inclusions N _Pb is 10,000 / mm ^{2 .} the above.

When the Pb content is less than 0.15%, the preferred cooling rate RC is 20 ° C / min or more, and the preferred molten steel flow rate VE is 20 cm / s or more. When the Pb content is less than 0.15%, the number N _{Pb of the} fine Pb inclusions is 10,000 pieces/mm ² or more, but most of them may not reach 15,000 pieces/mm ² . In this case, in order to make the total number TN of finely-cured inclusions 15000/mm ² or more, it is preferable to form a large amount of fine MnS inclusions. When the cooling rate RC is 20° C./min or more and the molten steel flow rate VE is 20 cm/s or more, fine MnS inclusions are generated in a large amount during hot working. Therefore, the total number TN of the finely-cured inclusions can reach 15,000 pieces/mm ² or more, and more excellent machinability can be obtained.

Further, when the processing start temperature is 1000 ° C or more, the extension of the coarse and fast-cut inclusions during the hot working can be suppressed.

The processing start temperature can be measured by, for example, a radiation thermometer disposed on the inlet side of an inter-heat processing apparatus (block rolling mill, continuous rolling mill, hot forging machine, etc.).

[Examples]

Lead-containing fast-cut steels were produced in a variety of chemical compositions and manufacturing conditions, and their machinability was evaluated.

〔experiment method〕

First, molten steel having test numbers 1 to 25 having the chemical composition shown in Table 1 was produced.

The material was produced by continuous casting using molten steel (the cast sheet section was 220 × 220 mm). The cooling rate RC (° C/min) at the time of casting of the steel of each test number is shown in Table 1. The cooling rate RC of each test number is obtained by first calculating the interval between the dendrite arms twice and calculating according to the above formula (3). Further, in continuous casting, electromagnetic stirring is performed on the molten steel in the mold. The molten steel flow rate VE (cm/s) of each test No. in the electromagnetic stirring was as shown in Table 1.

For each material of the test number, hot intercalation was performed to produce a round bar having an outer diameter of 50 mm. Each of the hot intercalations is one of performing block rolling, extension rolling, and hot forging. The processing start temperature T (° C.) was measured in the first hot work of each test number. The processing start temperature T of each test number is shown in Table 1.

For each test number, after performing various heat processing, the surface of the material after the heat processing was observed to confirm the presence or absence of cracks. If a crack occurs, the test of the test number is terminated.

[Fast cutting inclusion observation test]

A test piece for tissue observation was taken from the round bar of each test number. Among the surfaces of the test piece, a cross section including the center line of the round bar in parallel with the long axis direction of the round bar (in other words, the rolling direction or the extending direction) is defined as a "mirror surface". According to the above method, the number of fine Pb inclusions N _Pb (pieces/mm ² ) and the total number of finely-cut inclusions TN (pieces/mm ² ) were determined in the mirror surface. The number N _Pb of fine _Pb inclusions and the total number TN of finely-curved inclusions of each test number are indicated in Table 1.

[bit perforation test]

The bit penetration test was used to judge the machinability of the steel of each test number. The drill bit perforation test was performed for a round bar of each test number, and a plurality of holes of 15 mm depth were continuously formed using a drill at an arbitrary cutting speed. Then, the highest cutting speed VL1000 (m/min) at which the integrated hole depth reaches 1000 mm can be cut (in other words, the number of holes punched at a depth of 15 mm can be 67 or more) can be obtained.

Specifically, using the NACHI (trade name) system A drill with a diameter of 5 mm. The amount of protrusion of the drill was set to 60 mm, the feed amount was set to 0.33 mm/rev, and a commercially available water-soluble cutting oil was used for the perforation. The direction of the perforation is selected to be perpendicular to the direction of the long axis of the round bar (transverse direction). The drilling process is continuously repeated until the bit is melted or damaged, and the cutting speed VL1000 is obtained. The larger the cutting speed VL1000 is, it means that a large number of holes can be drilled at a high speed, so that it is judged that the tool life is excellent and the machinability is high.

[Flange cutting test]

The surface roughness of the steel of each test number after cutting was evaluated by the flange cutting test shown in FIG. 5 and FIG. The flange cutting test is performed while rotating the round bar 30 along the axis. The projecting tool 20 cuts the surface of the round bar 30, and sequentially forms the grooves G1 to G10 as shown in Fig. 5B. Specifically, the cutting tool 20 is advanced in the radial direction of the round bar 30 to form the groove G1. Then, as shown by the arrow in FIG. 5B, the cutting tool 20 is moved backward in the radial direction of the round bar 30, and then moved by a predetermined distance in the axial direction of the round bar. Then, the cutting tool 20 is again advanced in the radial direction to form the groove G2. Then, similarly, the grooves G3 to G10 are formed in this order. After the groove G10 is formed, the cutting tool 20 is moved to the position of the groove G1 again, and the groove G1 to the groove G10 are again subjected to groove processing. After 200 grooves were processed (20 grooves were processed for each of the grooves G1 to G10), the surface thickness of the bottom surface of the groove G10 was evaluated.

The material of the cutting tool 20 is equivalent to the Japanese industrial specifications. SHK57 (JIS specification) has a rake angle of 20° and a relief angle of 6°. The cutting speed of the cutting tool 20 at the time of groove processing was 80 m/min, and the feed amount was 0.05 mm/rev. A commercially available water-insoluble cutting oil is used for cutting.

The surface thickness was measured according to the following method. needle The maximum height Rmax (μm) was measured in accordance with Japanese Industrial Standards (JIS) B0601 (1972) using the stylus type surface roughness meter on the bottom surface of the groove D10 after the groove processing. The smaller the maximum height Rmax, the better the machinability is judged.

〔test results〕

The test results are shown in Table 1. The "Yes" in the "Processing Crack" column in Table 1 means that it is confirmed that cracks have occurred after the hot room processing; "None" means that no cracks have been confirmed. In the column "N _Pb ", the number of fine Pb inclusions indicating the respective test numbers is N _Pb (pieces/mm ² ). In the "TN" column, the total number TN (pieces/mm ² ) of the finely-curved inclusions indicating the respective test numbers is indicated. In the "VL1000" column, the cutting speed (m/min) indicating each test number obtained by the drill bit perforation test is indicated. In the column "Rmax", the maximum height Rmax (μm) of the surface of each test number obtained by the flange cutting test is indicated.

It can be seen from Table 1 that the test number is 1 to 15, the chemical composition is appropriate, the cooling rate in the casting process is RC (°C/min), the flow rate of the molten steel is VE (cm/s), and the processing in the hot working process The starting temperature T (°C) is also suitable. Therefore, the number N _Pb (pieces/mm ² ) of the fine Pb inclusions in the steel is 10,000 pieces/mm ² or more, and the total number TN (pieces/mm ² ) of the finely-grounded inclusions is 15,000 pieces/mm ² or more. Therefore, the cutting speeds VL1000 of the test numbers 1 to 15 are each high, and are all 130 m/min or more. In addition, the maximum height Rmax of the test numbers 1 to 15 is also small, and each is 14.5 μm or less.

Test No. 16, the chemical composition is suitable, the cooling rate RC is in the range of 15 to 30 ° C / min, the molten steel flow rate VE is 10 to 40 cm / s, and the processing start temperature T is 1000 ° C or more. Therefore, the cutting speed VL1000 is 130 m/min or more, and the maximum height Rmax is also 14.5 μm or less. However, the Pb content is less than 0.15% and the cooling rate RC is less than 20 ° C/min. Therefore, in Test No. 16, the number N _Pb (pieces/mm ² ) of the fine Pb inclusions in the steel is 10,000 pieces/mm ² or more, but the total number of finely-cut inclusions TN (pieces/mm ² ) is not Full 15000 / mm ² . Therefore, the one of the cutting speed VL1000 and the maximum height Rmax is worse than the values of the test numbers 1 to 15.

On the other hand, test number 17, although the chemical composition is Appropriate, but the cooling rate RC during the casting process is too fast. Therefore, it was confirmed that the material after the first hot working was cracked.

Test No. 18, although the chemical composition is appropriate, the cooling rate RC is too slow. Moreover, the molten steel flow rate VE is also too slow. Further, the processing start temperature T is less than 1000 °C. Therefore, the number N _Pb (pieces/mm ² ) of the fine Pb inclusions in the round bar and the total number TN (pieces/mm ² ) of the finely-curved inclusions are too small. As a result, the cutting speed VL1000 is too small and the maximum height Rmax is too high.

Test No. 19, although the chemical composition is appropriate, the molten steel flow rate VE is too slow. Therefore, the number of fine Pb inclusions N _Pb (pieces/mm ² ) is too small, and the maximum height Rmax is high.

Test No. 20 is that the oxygen content is too low. In addition, the molten steel flow rate VE is too slow. Therefore, the number of fine Pb inclusions N _Pb (pieces/mm ² ) is too small, the cutting speed VL1000 is small, and the maximum height Rmax is also high.

Test No. 21, although the chemical composition is appropriate, the cooling rate RC and the molten steel flow rate VE are too slow. Therefore, the number of fine Pb inclusions N _Pb (pieces/mm ² ) is too small, and the maximum height Rmax is high.

Test No. 22, the N content was too low. Therefore, the maximum height Rmax is large and the machinability is very low. Considered because the N content is very low, Therefore, the ductility of the substrate becomes too high.

Test No. 23, although the chemical composition is appropriate, the cooling rate RC and the molten steel flow rate VE are too slow. Therefore, the number of fine Pb inclusions N _Pb (pieces/mm ² ) is too small, the cutting speed VE is small, and the maximum height Rmax is high.

Test No. 24, although the chemical composition, the cooling rate RC, and the molten steel flow rate VE are appropriate, the processing start temperature T is less than 1000 °C. Therefore, the number of fine Pb inclusions N _Pb (pieces/mm ² ) is too small, the cutting speed VE is small, and the maximum height Rmax is high.

Test No. 25, although the chemical composition, the molten steel flow rate VE, and the processing start temperature T were appropriate, the cooling rate RC was too slow. Therefore, the number of fine Pb inclusions N _Pb (pieces/mm ² ) is too small, the cutting speed VE is small, and the maximum height Rmax is high.

Although the embodiments of the present invention have been described above, The above embodiments are merely illustrative of examples for implementing the invention. Therefore, the present invention is not limited to the above-described embodiments, and may be practiced without departing from the spirit and scope of the invention.

40‧‧‧Pb inclusions

Claims (9)

A lead-containing fast-cut steel containing C: 0.005 to 0.2%, Mn: 0.3 to 2.0%, P: 0.005 to 0.2%, S: 0.01 to 0.7%, Pb: 0.03 to 0.5%, and N by mass% : 0.004~0.02%, and O: 0.003~0.03%, the rest is composed of Fe and impurities. The number of Pb inclusions in the steel corresponding to a diameter of 0.01~0.5μm is 10000/mm ² the above. The lead-containing fast-cut steel according to claim 1, wherein the number of Pb inclusions corresponding to a circle diameter of 0.01 to 0.5 μm in the steel, and the MnS inclusion corresponding to a circle diameter of 0.01 to 0.5 μm. The total amount of the objects is 15,000 pieces/mm ² or more. The lead-containing quick-cut steel according to the first aspect of the invention, which comprises one selected from the group consisting of Cu: 0.5% or less, Ni: 0.5% or less, and Sn: 0.5% or less. Two or more kinds are substituted for a part of the aforementioned Fe. The lead-containing quick-cut steel according to the second aspect of the invention, which contains one selected from the group consisting of Cu: 0.5% or less, Ni: 0.5% or less, and Sn: 0.5% or less. Two or more kinds are substituted for a part of the aforementioned Fe. The lead-containing quick-cut steel according to the first aspect of the invention, which contains one or more selected from the group consisting of Te: 0.2% or less and Bi: 0.5% or less, in place of a part of the above-mentioned Fe. . The lead-containing quick-cut steel according to the second aspect of the invention, which contains one or more selected from the group consisting of Te: 0.2% or less and Bi: 0.5% or less, in place of a part of the above-mentioned Fe. . The lead-containing quick-cut steel according to the third aspect of the invention, which contains one or more selected from the group consisting of Te: 0.2% or less and Bi: 0.5% or less, in place of a part of the above-mentioned Fe. . The lead-containing quick-cut steel according to claim 4, which contains one or more selected from the group consisting of Te: 0.2% or less and Bi: 0.5% or less, in place of a part of the above-mentioned Fe. . The lead-containing quick-cut steel according to any one of the first to eighth aspect of the present invention, which is characterized by containing a group consisting of Cr: 0.5% or less and Mo: 0.5% or less. One or more of them are substituted for a part of the aforementioned Fe.